Paired vowels are: unrounded • rounded
A close vowel, also known as a high vowel (in American terminology), is any in a class of vowel sound used in many spoken languages. The defining characteristic of a close vowel is that the tongue is positioned as close as possible to the roof of the mouth as it can be without creating a constriction. A constriction would produce a sound that would be classified as a consonant.
The term "close" (//, as in the opposite of "far") is prescribed by the International Phonetic Association. Close vowels are often referred to as "high" vowels, as in the Americanist phonetic tradition, because the tongue is positioned high in the mouth during articulation.
In the context of the phonology of any particular language, a high vowel can be any vowel that is more close than a mid vowel. That is, close-mid vowels, near-close vowels, and close vowels can all be considered high vowels.
The six close vowels that have dedicated symbols in the International Phonetic Alphabet are:
(IPA letters for rounded vowels are ambiguous as to whether the rounding is protrusion or compression. However, transcription of the world's languages tends to pattern as above.)
There also are close vowels that don't have dedicated symbols in the IPA:
Other close vowels can be indicated with diacritics of relative articulation applied to letters for neighboring vowels, such as ⟨i̠⟩ or ⟨ɪ̝⟩ for a close near-front unrounded vowel.